Chemical enrichment mechanisms in Omega Centauri:

clues from neutron-capture elements

V. D’Orazi(INAF –Osservatorio Astronomico di Padova)

R. Gratton, S.Lucatello (INAF –Padova)A.Bragaglia, E.Carretta, E.Pancino (INAF –Bologna)

C.Sneden (University of Texas at Austin)

Santa Cruz, 12 July 2011

“A Simple Stellar Population is defined as an assembly of coeval, initially chemically homogeneous, single stars ..

Four main parameters are required to describe a SSP, namely its age, composition (Y,Z), and the initial mass function

..In nature the best example of SSPs are stellar clusters” (Renzini & Buzzoni 1986).

THIS TRADITIONAL PERSPECTIVE IS NOW PROVEN

TO BE TOO SEMPLISTIC….

Photometry Spectroscopy

Globular Clusters ARE NOT Simple Stellar Populations

Photometry

Piotto et al. (2007)

ω Cen

Lee et al. (1999)

Pancino et al. (2000)

NGC 2808

NGC 1851

Bedin et al. (2004)

Milone et al. (2008)

Spectroscopy

Lick-Texas group (from Ivans et al. 2001)

Since ’70s anti-correlations between light elements

(C, N, O, Na, Mg, Al) the abundances of C, O, Mg are depleted where those of N, Na, Al are enhanced

Cohen (1978); Peterson (1980); Norris (1981)

Marino et al.(2008, 2009)

M4

M22

Carretta et

al. (2009a)

All the GCs show the

Na-O anti-correlation

the second generation is

always PRESENT

P=primordial FG(share the same chemical composiiton of field stars with same [Fe/H])

I=Intermediate SG

E=Extreme SG

(high Na, low O)

A PREVIOUS GENERATION of stars which synthesized in their interiors p-capture elements are RESPONSIBLE for these

chemical signatures in GC stars

HOT hydrogen burning, where the ON, NeNa, and MgAl chains are operating - the ON reduces O, the NeNa increases Na

(T ~ 30 million K), while the MgAl produces Al (T~65 million K)

IM-AGB stars (4 – 8 M) experiencing Hot Bottom Burning

(e.g., Ventura+ 2001)

Winds of Fast Rotating Massive Stars

(e.g., Decressin+ 2007)

Still debated……

GCs are homogeneous concerning Fe-peak and the heavy alpha-elements

(e.g. Ca, Ti)

Light element variations (CH-CN, O-Na, Mg-Al)

Heavy elements (Z>30)

little star-to-star variation within GCs (Armosky+ 1994; James+ 2004)

Analysing a sample of ~1000 RGB GC stars (in 15 GCs) we found that [Ba/Fe]

does not significantly vary (D’Orazi+ 2010)

see Roederer (2011) for variation in r-process elements

A “typical” GC: NGC 6752

No variation in [Fe/H] +

Heavy elements

O-Na + Mg-Al anti-correlation

Li-O correlation (but the slope is not one)

Shen+2010

Carretta+2009b

However…..

NGC 7078 (M15) variation in r-process elements

δ[Eu/Fe]~0.5 dex (Sneden+1997; Sobeck+2011)

NGC 6656 (M22) δ[Fe/H]~0.15 dex positively correlated

with δ[Ba,Y,Zr/Fe]~0.4 dex (e.g., Marino+2009)

NGC 1851 variation in δ[Fe/H] from 0.06 to 0.25

(Carretta+2010a; Yong & Grundahl 2008)

δ[Ba/Fe] from ~0.6 dex to > 1 dex

(Carretta+2011; Villanova+2011; Yong & Grundahl 2008)

NGC 6715 (M54, Sgr dSph) δ[Fe/H] = 0.19 dex (Carretta+ 2010b)

NGC 5139 (Omega Centauri)

The most massive GC of our Galaxy: (~2.5x106 M, van de Ven+2006)

Photometric investigations:

(e.g. Wooleey 1966; Anderson 1997;Pancino+2000; Bedin+2004; Sollima+2005)

High-resolution spectroscopic study:

Norris & Da Costa (1995); Smith+(2000); Johnson & Pilachowski (2010); Marino+(2011)

Bellini+2010 Smith+2000

Johnson & Pilachowski (2010)more than 800 RGB stars

1. [Fe/H] variation from ~-2 to ~-0.6

2. alpha-elements + Fe-peak match the SN II

abundance pattern

3. Variation in O, Na, Al > 0.5 dex

(anti-correlated abundances, with the exception of the most metal-rich stars)

4. No significant contribution from SN Ia

5. Strong increase in [La/Fe] vs [Fe/H]

(not accompained by similar trend for [Eu/Fe]

low-mass AGB contributions

Marino et al. (2011) > 300 RGB stars

“The weak component at lower metallicity could produce heavier elements, up to Ba and La”

main component

(1-4 M AGB) responsible for elements beyond Zr (A>90)

weak component

(massive AGB stars) in solar system A<90

To test this prediction Lead (Pb) abundances

In the s-process, Pb can only be produced by the main component

D’Orazi+ (2011, subm.to A&A)

The sample

12 RGB stars

Spanning a range in metallicity from

[Fe/H]=-1.92

to [Fe/H]=-0.46

UVES@VLT spectra

(R~45,000)

λ=3600 – 4600 Å

and for two stars

also λ=5400 – 8900 Å

Pb, Y, Zr, La, Ce, Eu, and the C+N+O sum

Analysis

Pb I lines:3683 Å

4058 Å

YII line 4398 ÅZrII line 4208 Å

La II lines:4086 Å 4322 Å

Ce II lines: 4073 Å 4349 Å

Eu II line :4129 Å

CH G-band at 4300 Å

CN violet band at 4215 Å

We detected the hint for a Pb production occurring at [Fe/H]~-1.7 dex

At variance with Smith+(2000)

We found that the variation of light s-process

(first-peak) elements, i.e. Y, Zr

is LARGER than the one of

heavy (second-peak), here La, Ce.

We have to discard the most metal-poor stars, since they have a mostly r-process nucleosynthesis

Δ [<Y,Zr>/Fe]=0.7 dexΔ[<La,Ce>/Fe]=0.5 dex

The C+N+O sum

1. <[C/Fe]>=-0.51±0.10 (NDC95)

2. metal-poor stars are both

N-rich and N-poor, while metal- rich ones are ONLY N-rich

N-O anti-correlation

except for the most metal-rich

The C+N+O sum moderately raises up to [Fe/H] ~ -1.5 dex and then remains CONSTANT

The [Pb/ls] ratio varies

more than the [hs/ls]

The Pb variation

rules out the weak

component as main

mechanism for s-process

element production:

In Omega Cen the main component is at work

Weak component

1) Pb production

2) [Cu/Fe] costant as a function of [Fe/H] (e.g., Cunha+ 2002)

3) Theoretical studies also argue

that at lower metallicities the weak component can not produce significant amounts of elements heavier than Zr

see Raiteri+ (1992); Pignatari & Gallino (2008)

We agree with the Smith’s conclusion that low-mass AGB (main component) are responsible for the s-process element production in

Omega Cen..

An alternative scenario…

Intermediate-mass AGB stars (5-8 M) production of s-process elements

through 22Ne(α,n)25Mg

(Marino+2011)

(the same stars responsible for the light element pattern !!)

BUT: (1) Negligible production of heavy s-process elements, like e.g. Ba, La, Ce

(Pignatari & Gallino 2008)

(2) Overproduction of Rb with respect to Zr

(Garcia-Hernandez+2006)

Only for one star (#60073) Rb abundance (@7800A) [Rb/Zr]=-0.65

However…..

Smith+(2000) concluded that ~ 1.5 Msun AGB model better reproduce the observed pattern

>1Gyr difference between the different stellar generations in Omega Cen

Our results:

• More production of light s-process elements with respect to the heavy ones

• Pb production but NOT as larger as the one of light s-process elements

neutron exposures quite small, few thermal pulses : LARGER MASS (!!!)

we suggest the main component is peculiar in this GC and slanted towards the higher mass, i.e. about 3.5/4 Msun.

Note also that the CNO sum slightly raises with metallicity

Drastic reduction in the age difference from >1 Gyr to several Myrs